Direct bonding refers to the uniting of two bodies without an intermediate to act as a sealant. The direct bonding of an electrically conductive metal, such as silicon, to an insulator, such as a glass, is variously known as anodic bonding, electrostatic-bonding, or field-assisted bonding. It is also sometimes referred to as Mallory bonding, since the process was developed by personnel at P. R. Mallory & Co., Inc.
Direct bonding of a glass to a metal, such as silicon, is a low temperature process that is carried out below the glass softening point. It involves bringing a glass body and a metal body into close contact, and applying an electric potential across the two bodies. An electric current passes through the bodies and an electrostatic field is created, thereby effecting bonding of the bodies.
Electrostatic bonding has found a variety of applications where bonding at a low temperature is critical, or at least highly desirable. In particular, it avoids the distortion that tends to occur when a glass softens during fusion sealing. It is also very useful where metal elements are present that may be degraded or damaged by excessive heating.
Currently, there is considerable interest in using the technique in the fabrication of pressure sensors and accelerometers embodying insulated silicon elements. Among the important properties required of an insulating glass to be used in this way are:
1. An expansion match to silicon. This generally means a glass having a coefficient of thermal expansion (CTE) of 32-40.times.10.sup.-7 /.degree. C., preferably 34-38.times.10.sup.-7 /.degree. C.
2. Mild bonding conditions, that is, voltages below 1,000 volts/mm of glass thickness and a bonding temperature below 440.degree. C.
3. Reasonable glass etchability rate, e.g., removal of at least 100 mg/cm.sup.2 of glass in 20 minutes with 48% by weight HF.
Various glasses, including borosilicates, aluminosilicates and quartz, have been suggested for use in electrostatic bonding to different metals. The process was developed using a borosilicate glass, Corning Code 7740, in conjunction with silicon. In spite of problems, it is still current practice to use either Corning Code No. 7740 or Code No. 7070, both borosilicates, in electrostatic bonding to silicon.
Code 7740 does not provide a good expansion match to silicon and etches at a relatively slow rate. These problems are ameliorated when Code 7070 is used, but bonding has proven difficult. Thus, at least 4,000 volts per mm of glass are required together with a temperature of 450.degree. C. It is apparent that a glass having improved properties for use in electrostatic sealing to silicon would be very desirable. It is a basic purpose of my invention to meet this need.